Sunday, December 02, 2007

About 250 million years ago...

About 250 million years ago, some disaster wiped out 90 percent of life on Earth, a cataclysm known as the Permian-Triassic extinction. Around the same time, volcanoes a million times bigger than Mount Saint Helens erupted, spewing enormous clouds of dust and gas into the sky and covering the ground with 2 million square miles of molten lava. Did the volcanic eruptions cause the extinctions? The answer depends on which event occurred first and how long each took—and right now, scientists just don’t know.

Such is the case with most of the big questions about the history of the Earth, says Paul Renne, director of the Berkeley Geochronology Center: “Often, our arguments about causality depend on timing.” That’s why he and hundreds of other scientists around the world have joined Earthtime, a 10-year endeavor to nail down the sequence of past events on Earth by refining scientists’ techniques for measuring deep time. The project was started by Sam Bowring, an expert in geologic time at MIT, and Douglas Erwin, a paleontologist at the National Museum of Natural History, who conceived of Earthtime during a transcontinental flight together a decade ago. “If we really want to understand the history of the Earth, we have to push our dating tools to their limits,” Bowring says.

Over the past 10 years, such tools have become astonishingly accurate and precise. Take radioisotope dating, which scientists do by measuring the relative abundance of certain forms of elements (like potassium-40) called isotopes and then using the known decay rates of those elements to calculate the age of the minerals in which they are found. In the 1970s, scientists using radioisotope dating could pinpoint the age of a 100 million-year-old rock to within a few million years. Today Bowring and others have whittled the uncertainty down to less than 100,000 years.

But a few glitches are preventing scientists from making the most of these improvements. First, separate labs using the same dating techniques employ slightly different materials and methods, leading to different results. This didn’t matter when measurements of deep time were rough. Now that the science is more exact, though, small inconsistencies due to experimental error can spark big disagreements. “It’s when we sharpen our tools that these discrepancies turn up more glaringly,” Renne says.

Earthtime’s goal is to synchronize the numerous watches worn by scientists who study deep time, then use them to create one superaccurate chronology of Earth’s past. To do this, Bowring and his colleagues have distributed a set of reference materials called standards (rock samples with known ages) and tracers (small quantities of isotopes with a known composition) to help make different labs’ results consistent. Until now, labs have used varying standards and tracers, which has led to differing results. Bowring hopes to eliminate such problems by giving everyone the same starting point.

Bowring and his fellow keepers of deep time are also searching for rock beds where scientists can test how well dates derived by different methods agree. And they are looking for new samples of ancient material to fill in details about ancient events. For instance, next year the ship JOIDES Resolution is scheduled to drill into the floor of the Pacific Ocean to extract rock cores that will span the period from about 53 million to 18 million years ago, a time of vast climate change.

If it works, Earthtime should enable scientists to study new, previously unappreciated aspects of Earth’s past. Erwin compares the endeavor to the Human Genome Project, in which scientists mapped the sequence of our genes. “The fundamental goal of the Human Genome Project wasn’t the genome itself—it was figuring out what our genes do,” Erwin says. “Our goal is to produce a better timescale so we can go out and start asking a whole new set of questions.”

Erika Check Hayden
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